THE ALKALINE-EARTH FAMILY
| SYMBOL | ATOMIC WEIGHT | DENSITY | MILLIGRAMS SOLUBLE IN 1 L OF WATER AT 18° | CARBONATE DECOMPOSES | ||
| SULPHATE | HYDROXIDE | |||||
| Calcium | Ca | 40.1 | 1.54 | 2070.00 | 1670. | At dull red heat |
| Strontium | Sr | 87.6 | 2.50 | 170.00 | 7460. | At white heat |
| Barium | Ba | 137.4 | 3.75 | 2.29 | 36300. | Scarcely at all |
The family. The alkaline-earth family consists of the very abundant element calcium and the much rarer elements strontium and barium. They are called the alkaline-earth metals because their properties are between those of the alkali metals and the earth metals. The earth metals will be discussed in a later chapter. The family is also frequently called the calcium family.
1. Occurrence. These elements do not occur free in nature. Their most abundant compounds are the carbonates and sulphates; calcium also occurs in large quantities as the phosphate and silicate.
2. Preparation. The metals were first prepared by Davy in 1808 by electrolysis. This method has again come into use in recent years. Strontium and barium have as yet been obtained only in small quantities and in the impure state, and many of their physical properties, such as their densities and melting points, are therefore imperfectly known.
3. Properties. The three metals resemble each other very closely. They are silvery-white in color and are about as hard as lead. Their densities increase with their atomic weights, as is shown in the table on opposite page. Like the alkali metals they have a strong affinity for oxygen, tarnishing in the air through oxidation. They decompose water at ordinary temperatures, forming hydroxides and liberating hydrogen. When ignited in the air they burn with brilliancy, forming oxides of the general formula MO. These oxides readily combine with water, according to the equation
MO + H2O = M(OH)2.
Each of the elements has a characteristic spectrum, and the presence of the metals can easily be detected by the spectroscope.
4. Compounds. The elements are divalent in almost all of their compounds, and these compounds in solution give simple, divalent, colorless ions. The corresponding salts of the three elements are very similar to each other and show a regular variation in properties in passing from calcium to strontium and from strontium to barium. This is seen in the solubility of the sulphate and hydroxide, and in the ease of decomposition of the carbonates, as given in the table. Unlike the alkali metals, their normal carbonates and phosphates are insoluble in water.